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The flow characteristics around the frontal face of a square cylinder with a frontal jet injection were experimentally studied in a closed-loop wind tunnel using the PIV technique. Four characteristic flow modes were identified based on the injection ratio: swinging jet, deflected oscillating jet, deflection jet, and penetrating jet. At low injection ratio, the flow mode denoted as the swinging jet is characterised by two recirculation regions and a four-way saddle point. At moderate injection ratio, the flow mode termed deflected oscillating jet is characterised by a recirculation region and a counterclockwise rotating vortex within the recirculation region. At moderately high injection ratio, the flow mode is denoted as deflection jet, where a clockwise rotating vortex appears in the flow field. At high injection ratio, the flow mode is termed as penetrating jet, with no vortex formation in the flow field. The strength of the turbulence intensities increases considerably with high injection ratios. The time histories of instantaneous velocity for non-injection case, swinging jet, and deflected oscillating jet modes show periodic oscillations, whereas those of deflection jet and penetrating jet modes do not show periodic oscillations. The study presents and discusses the time-averaged velocity vectors, and streamlines, vorticity contours, velocity properties, velocity time histories, and power spectral density function under different flow modes. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Drainage & Irrigation Machinery Engineering / Paiguan Jixie Gongcheng Xuebao is the property of Editorial Department of Drainage & Irrigation Machinery Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The objective of this study was to explore the higher-order turbulence statistics of flow and turbulent length scales around a complex pier (CP) and compare the results with those for a simple pier (SP). The velocity data were recorded in a laboratory flume using particle image velocimetry (PIV). The PIV data were analysed to estimate the velocity fluctuations of third order, turbulence production and dissipation rates, turbulent length scales and the contributions of burst–sweep events to the total Reynolds shear stress around the piers mounted on a rigid flat surface. The skewness and advection coefficients indicated a more asymmetric distribution of velocity fluctuations for the SP than for the CP. Ejection and sweep events illustrated the dominance of similar strengths for an extended period, downstream of the piers. The upstream turbulent kinetic energy production was similar for both piers, while the upstream dissipation rate was higher for the CP. The length scales were greater at the upstream for the case of the SP while, downstream, they were greater for the CP. The findings of this work demonstrate the importance of a pile cap in restricting downward-moving flow by showing a lower magnitude of scour-inducing turbulence parameters for the CP as compared with the SP. [ABSTRACT FROM AUTHOR]

The studies on vascular flows have increased in the last decade. In this work; we have focused on the effects of flow model and blood rheology on hemodynamics for a real-subject scan using Computed Tomography Angiography (CTA) during numerical solutions. Various vascular flow studies using Newtonian or non-Newtonian blood models were presented in the literature with laminar or turbulent flow assumptions. In this study; six different turbulent models (Realizable k- ε , Standard k- ε , SST k- ω , Standard k- ω , Transition k-kl- ω , Transition SST) were compared to laminar flow to show whether turbulent flow solution is necessary. Blood rheology was investigated by using five different non-Newtonian models (Carreau, Herschel–Bulkley, Carreau–Yasuda, Casson, Power-Law) in addition to Newtonian model to indicate whether non-Newtonian blood assumptions is necessary. The In vivo boundary conditions were utilized by the UDF code which defines the real-patient cardiac cycle obtained by Echocardiography (ECHO) to present hemodynamics in the study. The results show that laminar flow well matched with the four turbulent models and two models shows by 4.8% and 19.5% differences in Wall Shear Stress (WSS) according to laminar flow. When the blood rheology was investigated, results revealed significant differences in WSS by 25.7%, 8.7%, 22.4%, 12.3%, and 32.5% for the non-Newtonian models in the given order, respectively, compared to Newtonian assumption. It concluded that laminar flow solution could be effective instead of solving turbulent flows in terms of computational cost, however, non-Newtonian blood effects could be considered to determine critical hemodynamics levels in a normal aortic arc. [ABSTRACT FROM AUTHOR]

This research investigates the cavitation wear phenomena in a direct-acting pressure relief valve (PRV), where the poppet of the PRV is subjected to unseat at over-pressure conditions of the hydraulic system. The poppet unseat results in to sudden fall in system pressure and hence, the expansion and burst of pre-existing tiny air bubbles and thus the cavitation. To reduce the occurrence of cavitation, three different poppet shapes i.e. chamfered conical, blunt, and spherical heads is proposed for replacement over the existing conical head. To investigate the effect of different poppet shapes, the CFD simulation of the simplified 2-D fluid flow geometry is considered and the same is validated through experiments. Further, the validated model is used to investigate the cavitation parameters during the PRV operation at different opening conditions. The results showed that the blunt head PRV poppet offers comparatively lesser cavitation intensity for hydraulic systems subjected to frequent over-pressure conditions. [ABSTRACT FROM AUTHOR]

The aerodynamic forces on road vehicles and flow structures around them result from complex interactions between fluid and structure. Ahmed body is a simplified car model created to demonstrate and simplify the flow around real-size ground vehicles. In this study, the flow structure on the wake region of Ahmed body with different slant angles (ϴ=15°, 25°, and 35°) was investigated. Experimental studies were conducted in a water channel by Particle Image Velocimetry (PIV). The freestream velocity was set to 0.2 m/s, and the Reynolds number defined by the characteristic length of the Ahmed body was 4.16×104 for Computational Fluid Dynamics (CFD) and PIV experiments. CFD simulations were performed using three different turbulence models: realizable k-ϵ, RNG k-ϵ, and SST k-ω, and the results were compared to experiments. The results are presented with different flow features such as time-averaged velocity vectors and velocity contours, streamline topology, vorticity, and Turbulence Kinetic Energy. The closest results to the experiments were obtained by the SST k-ω turbulence model for all slant angles of the Ahmed body. In addition, the drag coefficient is found to be 0.37 for all slant angles analyzed by SST k-ω turbulence models, which are also close to the results in the literature. [ABSTRACT FROM AUTHOR]

This investigation deals with the turbulence characteristics for flow through degraded channel beds consisting of uniform and bimodal sediment particles. Three-dimensional velocity was measured throughout the depth at three different streamwise locations over a degraded bed. The velocity data were then processed to observe the spatial distributions of the longitudinal and vertical velocity components in vector form, turbulence indicator and length scales at different locations. The main intent of the present study was to observe the effect of bed degradation on the intermittency and anisotropy in higher order scaling exponents using the structure functions methodology. The extended self-similarity (ESS) technique was employed to estimate the scaling exponents from higher order structure functions of streamwise velocity component. The results show that below the bed-level (level of the bed surface measured before the degradation started), the scaling exponents deviate more from the theoretical values than that above the bed-level indicating higher intermittency of scaling exponents in the vicinity of the bed. Furthermore, data were analyzed to observe the scaling exponents from higher order mixed structure functions. To this end, the SO (3) symmetry decomposition was utilized to differentiate the isotropic and anisotropic parts of the scaling exponents. The anisotropic scaling exponents obtained from higher order mixed structure functions possess higher values than that of isotropic part. The experimental findings also specify that as one moves close to the degraded bed, the scaling exponents obtained from the higher order mixed structure functions deviate more significantly. However, according to the present experimental results, no significant differences were observed in the values of the scaling exponents and intermittency when they were compared between uniform and bimodal sedimentary beds. It is expected that the results of the present study will essentially lead to new contributions to the existing knowledge in the field of turbulence through degraded and scoured channel beds. [Display omitted] • Experimental results of flow through a degraded channel bed are presented focused on the scaling exponents and intermittency in turbulence. • Higher order streamwise structure functions and the corresponding scaling are presented for flow over a degraded bed. • Higher order mixed structure functions are calculated, and the corresponding isotropic and anisotropic scaling exponents are estimated. • The anisotropic scaling exponents possess higher values than that of isotropic part. [ABSTRACT FROM AUTHOR]

• Flow characteristics created by different curved blade impellers are investigated by SBES model. • All curved impellers reduce power consumption by reducing drag force rather than drag arm length. • Staggered fan-shaped blade of SFPDT are superior to enhancing TKE transport. • The swept-back blade structure of SPDT results in the lowest level of power number and TKE values. A high-performance impeller is crucial for enhancing material mixing in a stirred tank. In this work, the flow characteristics created by different curved blade impellers including staggered fan-shaped parabolic disc turbine (SFPDT), swept-back parabolic disc turbine (SPDT), asymmetric staggered parabolic disc turbine (ASPDT) and traditional parabolic disc turbine (PDT) in stirred tanks are investigated numerically by using the stress-blended eddy simulation (SBES) model with a sliding mesh approach. After successful validation of prediction accuracy, power characteristics, mean flow, turbulence characteristics, turbulent kinetic energy (TKE) transport, and trailing vortices behaviors of SFPDT, ASPDT, SPDT and PDT in stirred tanks are systematically evaluated. The results show that ASPDT leads to a significant asymmetric distribution in the axial direction for velocity, TKE and trailing vortices, but the radial jet reduces more severely in the radial direction. The fan-shaped geometry constrains the influence of asymmetric staggered structure on the TKE and trailing vortices distribution for SFPDT. The swept-back blade structure of SPDT results in the lowest power number and TKE values level. These results provide a foundation for the further development and application of high-efficiency impellers. [Display omitted] [ABSTRACT FROM AUTHOR]

• Applied particle velocimetry to study liquid–solid two-phase flow. • Developed a denoising algorithm based on circularity and grayscale thresholds. • Analyzed flow characteristics of macroscopic velocity and microscopic vorticity. • Established a Lagrangian distribution for particle velocity vectors. • Quantified the slip velocity between liquid and solid phases. The liquid–solid particle flow within a horizontal pipeline is a typical particle-laden flow. The interplay between loaded particles and carrier phase engenders significant complexities in the dynamic behavior of such flows. We investigated the dynamics of a liquid–solid particle flow featuring dilute, slightly buoyant, hundred-micron-sized spherical particles fully suspended in the turbulent flow in a pipe, where Re is 7038. Cases with solid volume fractions of 0, 2.67 × 10-4, 5.33 × 10-4, 8.00 × 10-4, 1.07 × 10-3 and 1.33 × 10-3 were considered in this study. Experimental measurement techniques were utilized to acquire images of particles in the two-phase flow across the entire flow field via optical field feedback. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) were employed to simultaneously obtain liquid-phase velocity fields and particle trajectories. This approach allowed for a comprehensive depiction of the velocity field of each phase. Subsequently, a detailed explanation and analysis of the flow characteristics of the liquid–solid particle flow were provided based on the distribution of macroscopic velocity fields, the microscopic vorticity field, particle velocity vectors, and velocity slip. As a result, it was found that with the addition of solid particles and an increase in volume concentration, the drag effect of the solid phase and the trend of accumulation near the lower wall caused a decrease in the liquid phase velocity profile and deformation of the parabolic shape. In the liquid–solid particle flow with a volume concentration of 1.33 × 10-3, compared to the single-liquid phase flow, the standard deviation of velocity in the central region increased from 0.0097 m/s to 0.0159 m/s, and in the near-wall region, it increased from 0.0257 m/s to 0.0347 m/s, representing increases of 1.54 times and 1.26 times respectively. The proportion of medium vortices increased from 17 % to 30 %, nearly doubling. This study actively explores the concurrent measurement and flow characteristics of liquid–solid particle flow. [ABSTRACT FROM AUTHOR]

In this research, the impermeable spur dike combined with grouped piles were arranged as two spur dike patterns (L and T), and the corresponding flow and turbulence characteristics in a rectangular open channel flow were investigated using a computational fluid dynamics (CFD) numerical approach based on the standard k - ε model. The patch lengths of the grouped pile were varied between 0.2 and 0.6 m around the impermeable spur dike, and the flow and turbulence behavior near the spur dike tip, in the mainstream, and on both banks of the channel were observed. When the patch length of grouped pile increased to 0.6 m, the increasing trend of mean streamwise velocity at the downstream was reduced up to 5% in the left bank, 52% in the right bank, and 8% in the mainstream. The tip of the spur dike turbulence intensity (T.I), turbulence kinetic energy (TKE), and mean streamwise velocity reduction were 19%, 35%, and 12%, respectively. The results revealed that both the spur dike patterns (L and T) are useful to protect the riverbanks and the spur dike tip suffering from higher velocities and turbulence effects comparing with the typically impermeable spur dike. [ABSTRACT FROM AUTHOR]

In the paper, a 3-D numerical model based on the olaFlow solver to study the vegetation effects on flow characteristics and turbulent structure as well as wave attenuation under combined regular wave-unidirectional current conditions. The results show that following currents can either promote or suppress wave attenuation within the velocity magnitude specified in the paper, whereas opposing currents always enhance wave damping. Moreover, the emergent canopy and a staggered configuration resulted in increased wave-height attenuation. The time-averaged velocity patterns are various for submerged and emergent scenarios with different configurations. Furthermore, the time-averaged velocity magnitude inside the canopy decreases as the wave height increases for following flow conditions, while it increases in opposing currents. The time-averaged turbulent kinetic energy (TKE) inside the canopy follows the same distribution pattern as that of time-averaged velocity, and the maximum values of time-averaged TKE for the submerged and emergent canopies appear at the free surface, and above, respectively. The flow field characteristics show different patterns for various wave-current combinations and vegetation configurations, which provides a microscopic perspective to understand the interaction of vegetation-wave-current. • A three-dimensional numerical model using cyclic boundary based on the OlaFlow solver was developed. • Time-averaged velocity and turbulent kinetic energy within submerged/emergent canopy present different patterns. • Microscopic flow fields characteristic within the vegetation in following and opposing currents are presented. [ABSTRACT FROM AUTHOR]

The turbulence characteristics in open-channel flow owing to a sudden change from smooth (upstream) to rough (downstream) bed are investigated experimentally using a Particle Image Velocimetry system. The upstream flow had a shear Reynolds number of 4.86, characterizing the hydraulically smooth flow, while the downstream flow had a shear Reynolds number greater than 70, characterizing the hydraulically rough flow. The Reynolds stress results reveal that for a given vertical distance, both the Reynolds shear and normal stresses in the downstream bed increase with the streamwise distance as compared to their upstream values. Their peaks appear at a distance of one-fifth of the flow depth from the bed. In addition, the bed shear stress enhances with the streamwise distance. The stress contours corroborate that the formation of a roughness-induced layer over the downstream bed thickens with the streamwise distance. The third-order correlations reveal that an arrival of slowly moving fluid streaks associated with an outward Reynolds stress diffusion prevails in the flow on the upstream bed, while an inrush of rapidly moving fluid streaks associated with an inward Reynolds stress diffusion governs the near-bed flow zone in the downstream bed. These results are in conformity with those obtained from the turbulent kinetic energy (TKE) fluxes and the bursting events. With regard to the TKE budget, the peaks of TKE production and dissipation rates appear near the downstream bed and are greater than those in the upstream bed. However, in the downstream bed, an enhanced negative TKE diffusion prevails near the bed. Article highlights: Turbulent flow field on a sudden change from smooth to rough bed converting the flow from smooth to rough. Formation of a roughness-induced layer over the downstream rough bed. Understanding the effects of a sudden change in bed roughness on the turbulence characteristics from the perspectives of their time-averaged spatial fields. [ABSTRACT FROM AUTHOR]

In this study, numerical simulations were performed to investigate the flow and turbulence characteristics of rectangular spur dikes with varying permeability using the Reynolds stress turbulence model developed by three-dimensional (3-D) numerical code FLUENT (ANSYS). In this research, both permeable and impermeable spur dikes were investigated with varying permeability (25%, 37%, 49%, 62%, and 74%) to show its effect on the flow structure. At zh=3.5 cm (where z is the maximum flow depth, and zh is mid of flow depth), 3-D flow velocities and depth-averaged velocities were measured in the horizontal plane. Different velocity profiles and contours, turbulent intensities (T.I), and turbulent kinetic energy (TKE) were analyzed at various selected positions. The findings indicate that an increase in the permeability of the spur dikes up to 74% had a reciprocal effect on the mean stream-wise velocity. Moreover, the recirculation regions created within the impermeable spur dike field decreased with increased permeability of spur dikes. The permeable spur dike head showed a significant decrease in T.I, and TKE compared to the impermeable spur dike. Therefore, to protect the spur dike head from severe turbulent flow during floods and reduce the spur dike field's recirculation region, a permeable spur dike is preferred. [ABSTRACT FROM AUTHOR]

The presence of vegetation changes the flow and thereby modifies the river bedform. This study explores the characteristics of the bedforms and their interactions with the overlaying flow through submerged vegetation. Different types of turbulent flows are generated in a laboratory flume, and the bed topography and turbulent statistical parameters are measured. Results indicate that the bed morphology is characterized by the horseshoe scour holes around individual plants and the gully-ridge pattern in the wakes. These bedform features do not significantly influence the hydrodynamics in the free shear flow. However, the horseshoe scour holes, the gully-ridge pattern and sand ripples intensify the vertical variation of the flow velocity and promote the vertical momentum exchange in the bed shear flow. Besides, the horseshoe scour holes, the gully-ridge pattern and sand ripples persist in the‘quasi dual-flow’regime. These bedforms are found to promote momentum exchange and inhibit turbulent sweeping within the canopy layer. Above the vegetation canopy, however, these bed features are found to inhibit momentum exchange and promote turbulent ejection. [ABSTRACT FROM AUTHOR]

Wind barriers are nowadays commonly placed on bridges to protect vehicles from adverse cross-wind effects. In addition to this beneficial influence, wind barriers may adversely affect the bridge dynamic stability. This is particularly exhibited for long-span cable-supported bridges. It is therefore the scope of the present study to analyze the effects of wind barriers on aerodynamic and aeroelastic characteristics of bridge-deck sections of long-span cable-supported bridges together with the respective flow characteristics around bridge-deck sections. The focus is on various arrangements of wind barriers, i.e., (1) wind barriers placed at the windward bridge-deck edge only; (2) wind barriers placed at the leeward bridge-deck edge only; and (3) wind barriers placed at both windward and leeward bridge-deck edges. This was carried out experimentally on small-scale models in a boundary layer wind tunnel. Three typical bridge-deck section models were studied, i.e., Great Belt (Denmark), Kao-Pin Hsi (Taiwan), and Golden Gate (United States). The galloping susceptibility of the bridge-deck sections in all arrangements of wind barriers proved to be the same, as is the case for the empty bridge-deck sections without wind barriers; i.e., from this point of view, wind barriers do not adversely bridge dynamic stability. However, in configurations with the windward wind barrier only, as well as both windward and leeward wind barriers, the flutter susceptibility of the bridge-deck sections increases substantially; i.e., the critical flow velocity for the bridge flutter decreased significantly in comparison with the respective empty bridge-deck sections. For the leeward wind barrier only, the flutter susceptibility of the bridge-deck sections did not change and remained the same as it was for the empty bridge-deck sections. The empty bridge-deck sections do not exhibit any significant change concerning their susceptibility to flutter for various turbulence levels of the incoming freestream flow. The flutter susceptibility of cable-supported bridges equipped with wind barriers is lower in more turbulent incoming flows. The bridge decks with wind barriers are more resilient to flutter in more turbulent winds. Shear layers that separate from the top of the wind barrier may have an important role in the self-excited lift force and the pitch moment and, consequently, the dynamic behavior of bridge decks. [ABSTRACT FROM AUTHOR]

The results of an experimental study on the turbulence characteristics in flow over and downstream of an isolated bedform are presented. The vertical profiles of time-averaged streamwise velocity, Reynolds stresses and turbulent kinetic energy (TKE) fluxes at different horizontal locations over and downstream of a bedform are shown to demonstrate their variations. The experimental results signify a flow acceleration, separation and deceleration over the stoss-side, at the crest and in the leeside of a bedform. In addition, in the wall-wake region (far downstream of the bedform), the defects (uninterrupted upstream value minus downstream value at a given vertical distance) in time-averaged streamwise velocity, Reynolds shear stress (RSS), turbulence intensities and TKE fluxes demonstrate the self-preserving characteristics in their individual vertical profiles when they are scaled by their individual peak defects. For scaling the vertical distance to achieve the self-preserving characteristics, the defect profiles of velocity were scaled by the half-width of peak velocity defect, those of RSS and turbulence intensities by the half-width of peak RSS defect and those of TKE fluxes by the half-width of individual peak TKE flux defects. The peak defects of all the parameters lessen, as one moves toward the downstream, recovering their uninterrupted upstream profiles. [ABSTRACT FROM AUTHOR]

We investigated the flow characteristics in a tank of H/T = 1.5 stirred by a novel multi‐blade combined agitator (MBC) by using time‐resolved particle image velocimetry and large eddy simulation approach. The predictions were assessed by Y+ values, Taylor microscale and power spectrum analysis, as well as experimental validation of velocity distributions. Results demonstrate that the MBC agitator can load the energy into the system effectively with a power number of 12.5 in a turbulent regime, resulting in improved axial and radial mass exchange. The upper and lower short blades produce an axial down‐flow in the top half and an axial up‐flow in the bottom half, respectively. Part of axial flows change to radial flows by the radial pumping of the long blades, meanwhile, the impingement of two axial flows improves the axial mass exchange. These flow characteristics lead to an obvious improvement in the turbulent kinetic energy distribution uniformity with higher turbulent intensity. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Drainage & Irrigation Machinery Engineering / Paiguan Jixie Gongcheng Xuebao is the property of Editorial Department of Drainage & Irrigation Machinery Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In the present study, flow around circular and staggered vegetation patches was investigated numerically. For turbulence modelling, the Reynolds‐averaged Navier–Stokes technique and Reynolds stress model were adopted. The numerical model was validated with the experimental data using varying vegetation density and flow velocities. The simulated results of mean stream‐wise velocities were in close agreement with the experimental results. The results show that the mean stream‐wise velocity in the downstream regions of vegetation patches were reduced, whereas the velocity in the free stream regions were increased. The influence of neighbouring and staggered vegetation patches on the flow was observed. The vegetation patches with larger nondimensional flow blockage (aD = 2.3, where a is the frontal area per volume of patches, and D is the diameter of vegetation patches) offered more turbulence when compared to the patches with a smaller flow blockage (aD = 1.2). Larger turbulence in the form of kinetic energy and turbulent intensity was recorded within the vegetation as well as the regions directly behind the patches. Negative Reynolds stresses were observed at the top of submerged vegetation. The turbulence characteristics peaked at the top of vegetation, that is, z/h = 1.0 (where z is the flow depth, and h is the vegetation height), which may be migrated vertically as the frontal area of the vegetation patch is increased. This high frontal area also increased stream‐wise velocity above the vegetation, leading to an increased variation in turbulence around the vegetation canopy. [ABSTRACT FROM AUTHOR]

• Visualize the influence of density and shape of roughness elements on flow. • The numerical simulation results confirm the conclusion of physical simulation. • Appearance of turbulence caused by the increase of inertia force is described. A single fracture is the basic unit of fracture medium, and the roughness of the fracture surface is an important factor influencing hydraulic characteristics of the flow in bedrock fracture. However, effects of the shape and density of roughness elements (various bulges/pits on rough fracture wall surfaces) on water flow in a single fracture have not been thoroughly understood. Thus the water flow in a single fracture with different shapes and densities of roughness elements was simulated by using Fluent software in this study. The results showed that the existence of roughness elements could enhance the flow resistance and cause nonlinear flow. The shape difference of roughness elements, i.e. the value variation of the included angle between the tangent direction of the beginning of roughness element and the streamline direction, could change their influence degree on the flow regime. Increasing the density of roughness elements could bring more violent turbulent, reduce the relative aperture of fracture, and decrease the discharge per unit width correspondingly. With the increase of flow velocity, Forchheimer equation could fit the relationship between average velocity and hydraulic gradient better than Izbash equation. In addition, the variation of flow regime in rough fracture from laminar flow to turbulent flow was a process of the gradual dominance of inertial force. [ABSTRACT FROM AUTHOR]

This study characterizes the mean and turbulent flow fields resulting from several large-scale downbursts simulated in the Wind Engineering, Energy and Environment (WindEEE) Dome at Western University. Detailed three-component velocity Cobra probe measurements are conducted for several Reynolds numbers and downburst height to diameter ratios. The wind speed records at a comprehensive number of heights and radial distances are analyzed using a moving average approach similarly employed in some full-scale campaigns. A reasonable averaging time is determined and the analysis of the turbulent flow is carried out using first and second order statistics and the similarities with full-scale data is presented. Moreover, Particle Image Velocimetry (PIV) measurements are carried out in order to investigate the dynamics of downburst vortices for the same Reynolds numbers and the same downburst height to diameter ratios as in the Cobra probe measurements. The structure and evolution of the primary downburst vortex from PIV are compared with existing full-scale data. • Characterizes mean and turbulent flow fields for large scale experimentally simulated downbursts. • Analyzes simulated downburst wind speed time histories similar to the approaches used for full scale downburst records. • Investigates turbulence characteristics relevant to wind loading of structures. • Primary vortex structure and its evolution for the simulated downbursts are compared with the available full scale records. [ABSTRACT FROM AUTHOR]

Highlights • Quantitatively study on flow characteristics of nanofluids. • Vortex number, vorticity and turbulent kinetic energy are all increased. • With the increasing of volume fraction, the turbulence becomes more intense. • Addition of nanoparticles in the base fluid can enhance the mass transfer. Abstract The conventional tests of heat transferring for nanofluids have confirmed that, except the increase of thermal conductivity, there also exists an additional strengthening mechanism responsible for the heat transfer enhancement. The strengthening mechanism is the changes in the flow characteristics due to the interaction between the nanoparticles and the base fluid, which is also the main factor that affects the heat transferring of nanofluids. To reveal the strengthening mechanism, it is necessary to study the flow characteristics further. The past visualization experiment just gives the flow patterns of the nanofluids, and cannot quantitatively reveal the enhancement degree of the fluids after adding the nanoparticles. In this paper, by using PIV (particle image velocimetry), the flow characteristics of SiO 2 -water nanofluids in two-dimensional backward step flow are quantitatively studied. The results show that, under the same Reynolds number, the number of vortexes, vorticity and turbulent kinetic energy increase by the addition of nanoparticles. With the increasing of nanoparticle volume fraction, the turbulence becomes more intense, and the flow characteristics are constantly changing. For the vol. 3% SiO 2 -water nanofluids, the number of vortexes increases by 3, the average vorticity increases by 22.1%, and the average turbulent kinetic energy increases by 368% compared with pure water, when the Reynolds number is 5000. Therefore, it can be concluded that the mass and momentum transfer in nanofluids is enhanced, resulting in the heat transfer enhancement. [ABSTRACT FROM AUTHOR]

Centrifugal fan in series is the core facility for pneumatic transportation. Its operation is required to be adjusted real time as the flow rate of transported materials changes. For optimisation performance, internal flow of a type 700 three-stage fan at rated rotation speed is analysed with FLUENT6.3 for various total pressure. Multiple reference frame (MRF) model is used for the impellers rotation in simulation, and Re-Normalisation group (RNG) κ-ɛ turbulence model and Roe-FDS flux type with first-order upwind difference scheme are used for calculation of compressible flow under various total pressure. Flow characteristics are obtained and differences of total pressure and velocity distribution in every impeller are analysed in different conditions, and velocity distribution on meridian plane as well as section of wind guide plates and volute are compared. Results show that there are many vortexes and backflows in fan with higher total pressure, and the lower pressure, the more smoothly flows, besides flow at inlet and outlet of impellers and in guide plates is seriously influenced by total pressure. Curves of P-Q and P-η at 4600 r/min are predicted based on simulation, which is helpful reducing the impact caused by off-design operation and improving efficiency.

Turbulent characteristics in wall-wake flows downstream of wall-mounted and near-wall cylinders are investigated. The distributions of the defect of streamwise velocity, Reynolds shear stress and turbulence intensities exhibit a certain degree of self-preserving characteristic when they are scaled by their respective peak defect values. For the velocity defect distributions, the vertical distances are scaled by the half-width of peak defect velocity. However, for the distributions of the defects of the Reynolds shear stress and the turbulence intensities, the vertical distances are scaled by the half-width of Reynolds shear stress defect. The peak defects of all the quantities reduce with longitudinal distance signifying the recovery of the upstream distributions of the individual quantities. The third-order correlations reveal that for the wall-mounted cylinder, a streamwise acceleration associated with a downward flux of streamwise Reynolds normal stress (SRNS) in the inner-layer of wall-wake composes sweeps and a streamwise deceleration associated with an upward flux of SRNS in the outer-layer forms ejections. On the other hand, for the near-wall cylinder, a streamwise deceleration associated with a downward flux of SRNS in the inner-layer of wall-wake flow and the gap flow produces the inward interaction events, while the outer-layer characteristic is similar to that of wall-mounted cylinder. The turbulent kinetic energy (TKE) budget in the wake flow demonstrates strong negative pressure energy diffusion in addition to a strong TKE dissipation and diffusion and that in the gap flow exhibits a minor positive peak of pressure energy diffusion and a minor negative peak of TKE diffusion. [ABSTRACT FROM AUTHOR]

In this study, a mathematical model for the Reynolds stress profiles in different regions of a submerged turbulent plane offset jet was developed. The two-dimensional Reynolds-averaged Navier-Stokes and the continuity equations were solved for the determination of the velocity and the Reynolds stress profiles within the jet layer in different regions of a submerged offset jet. The theoretical derivations involved a boundary layer approximation. The velocity profiles in the preattachment region obeyed an exponential function, whereas those in the impingement and wall jet regions followed a combination of a power function for the inner layer and an exponential function for the outer layer of the jet. The computed velocity and the Reynolds stress profiles corresponded well with the previous experimental results of the flow in submerged turbulent plane offset jets. [ABSTRACT FROM AUTHOR]

Characteristics of turbulent kinetic energy (TKE) fluxes and budget, conditional Reynolds shear stress (RSS) and Reynolds stress anisotropy in the near- and far-wake flows downstream of a wall-mounted vertical circular cylinder are studied. The TKE fluxes exhibit that the near-wake flow is characterized by an accelerating effect and the far-wake flow by a retarding effect. The TKE budget is explained by using the principle of the TKE conservation. The profiles of TKE budget show that the wall-shear layer of the wall-wake flow is governed by a high negative pressure energy diffusion rate in addition to a high TKE dissipation rate. The conditional statistics of velocity fluctuations were studied to examine the turbulent bursting events by using the quadrant analysis. The conditional RSS profiles illustrate that the near-wake flow is characterized by inward interactions, while the far-wake flow is characterized by sweeps. In the wall-wake flow, the mean duration of bursting events is shorter than their mean interval of occurrence. The Reynolds stress anisotropy reveals that the anisotropic invariant maps appear to be a closed looping trend in the near-wake flow and an open looping trend in the far-wake flow. However, all the profiles, for example, TKE fluxes and budget, conditional RSS and turbulence anisotropy, regain their uninterrupted upstream profiles with an increase in downstream distance. [ABSTRACT FROM AUTHOR]

Present work evaluates the double averaged turbulence characteristics of the sand bed channel subjected to the downward seepage through permeable bed. Measures of turbulent statistics are observed to increase with the application of downward seepage. The form induced stress in near bed has a reducing effect with no seepage and an increasing effect with seepage. The seepage increases the turbulent kinetic energy and turbulent intensities causing the bed particles to move rapidly. The quadrant analysis suggests that at near bed, the sweep events in flows with seepage are the main bursting events towards the Reynolds stress production, while ejection and sweep events in no seepage flow have almost equal contribution. The increase in sediment transport with seepage is caused by an increase in flow turbulence production and an associated decrease in turbulent kinetic energy dissipation and turbulent diffusion. [ABSTRACT FROM AUTHOR]

A numerical investigation of Reynolds number sensitivity on flow past a twin-box girder was performed using large eddy simulation (LES) with Reynolds numbers of 1e2 ≤ Re ≤ 1e7. The simulation results reveal that the flow characteristics around the twin-box girder show different Reynolds number sensitivity according to the flow region and the flow state. At the leading edge, the flow experiences laminar separation-laminar reattachment state (1.5e3 < Re < 6e3), laminar separation-turbulence reattachment state (6e3 ≤ Re ≤ 8e4), and turbulence separation-turbulence reattachment flow state (Re > 8e4). Around the gap, four models exist depending on Reynolds number: laminar closed cavity flow (Re < 1.6e3), laminar open cavity flow (1.6e3 ≤ Re < 5e3), wake model (5e3 ≤ Re ≤ 9e3), and turbulent open cavity flow (Re > 8e3). In the tail of the box girder, a regular Karman vortex is first generated, and then the lower shear layer transits to turbulence, resulting in the generation of separation bubbles at the lower and upper trailing edges. However, due to the upper trailing edge being blunter than lower trailing edge, the Reynolds number sensitive range of upper trailing edge (1e2-1e7) is much wider than lower trailing edge (1e2-2e4). [ABSTRACT FROM AUTHOR]

This experimental and analytical study investigates the double-averaged (DA) turbulent flow characteristics within an array of large gravel obstacles found atop a porous gravel bed. Analysis of the experimental data reveals that the DA streamwise velocity preserves the logarithmic law above the form-induced sublayer, while a linear law and a third-degree polynomial function apply within the form-induced and interfacial sublayers, respectively. The form-induced shear stress is 70% of the DA Reynolds shear stress (RSS) occurring at the virtual bed level. The DA turbulent kinetic energy (TKE) components, streamwise and vertical, attain their peak values at the obstacle crest level, while they diminish sharply below the virtual bed level. The fluxes of the TKE streamwise and vertical components, however, change their signs slightly below the crest level, indicating a changeover of the dominance of the bursting events. For the TKE budget, the TKE production, diffusion, and pressure energy diffusion rate terms attain their peak values at the crest level, while the TKE dissipation rate has its peak value at the virtual bed level. Third-order moments of velocity fluctuations follow the linear relationship, and their signs change slightly below the crest level. The quadrant analysis suggests that the sweep events are the governing mechanism at the near-bed flow region, while the ejection events become predominant with an increase in vertical distance. The quadrant plots of the form-induced velocity components display a pseudo-elliptical scatter within the interfacial sublayer and a small circular cluster above the form-induced sublayer. [ABSTRACT FROM AUTHOR]

Flow characteristics over dimpled surface are studied numerically for a fully developed turbulent channel. The results show that dimples can effectively activate the near wall turbulence and cause an increase of total drag. The dimple depth also plays an important role in enhancement of hear transfer. [ABSTRACT FROM AUTHOR]

In the paper it is theoretically proven that the topological structure of the airflow field in an airliner cabin is absolutely unstable. These are typical flow characteristics in the cabin. This also explains why in previous numerical simulations and experiments, discrepancies often arose in flow-direction results. Next, we used the fine LES with 22 million grid cells to reproduce the primary flow instability phenomenon in a symmetric cabin. However, the unsteady RANS (URANS) method with conventional turbulence models, such as the RNG k-ɛ turbulence mode, the Realizable k-ɛ turbulence model and the V2f turbulence model, cannot effectively simulate the instability of flow field in a symmetric cabin. So we developed an anisotropic model (BV2fAM) based on the idea of the V2f model. Three million grid cells are used in the BV2fAM simulation. The flow instability phenomenon is successfully reproduced in the numerical simulation of BV2fAM. [ABSTRACT FROM AUTHOR]

In the present investigation, the transient developments of flow and cavitation within an injector’s nozzle during the oil drainage process have been studied by numerical method for a high-pressure common-rail DI diesel engine, both the variation regulations of macro parameters (indicating flow characteristics and cavitation characteristics) and the distribution manners of important physical fields (indicating the cavitation evolution in the micro) have been obtained and analyzed. The obtained numerical results indicate that, during the oil drainage process, both mass flow rate and flow coefficient monotonously increase with declining variation rates, both averaged flow velocity and averaged turbulent kinetic energy also monotonously increase; however, to the curve of TKE-needle lift, there exist certain points give abrupt increase. The difference in TKE curve compared to averaged flow velocity is mainly attributed to the sudden variation of cavitation. Based upon the numerical results, the cavitation bubble will not be formed until the needle lift has been raised to a certain position due to the lower flow velocity and the lack of low (even negative) pressure zones. As needle rises, the primary bubbles are formed near the lower corner after nozzle’s entrance; but as needle further rises, the positions at where bubbles are formed have been transferred to the upper corner and then being blow downwards orifice as the increase of flow velocity. [ABSTRACT FROM AUTHOR]

The complex flow over three vertical drops (horizontal apron, adverse apron, horizontal apron with an end sill) were studied numerically and compared to experiments and empirical functions. In addition to flow depth, energy loss, and free-surface profiles, detailed velocity, pressure and shear stress distribution, which are hard to be measured experimentally, were investigated. The 2D numerical results on a 57,512-node grid, using standard k-ε turbulence model and standard wall function showed the best agreement with experimental results. The energy loss in the drop with an end sill was higher than the adverse apron drop and the latter more than the horizontal apron drop. The numerical length and velocity scales confirmed the experimental similarity profiles. However, due to dissipative nature of the turbulence model, the velocity gradient in the shear layer was numerically dispersed particularly near the pool end. The stagnation pressure and shear stress were computed numerically for the drops. The maximum stagnation pressure and shear stress occurred in the horizontal apron drop. [ABSTRACT FROM AUTHOR]

We carried out a twin-rudder performance test in a circulating water channel using particle image velocimetry (PIV). We set the distance between two single rudders to 0.5C, 0.75C, and 1.0C, where C is the chord length (C=100mm), and considered angles of attack of 10°, 20°, and 30°, respectively. A two-frame gray-level cross-correlation PIV method was used to obtain flow information. The twin-rudder model was constructed using a submerged NACA 0018 airfoil with a row arrangement. Throughout the experiments, the Reynolds number of the NACA 0018 airfoil was kept at 1.5×104. The measured results were compared with results of each case involving a single rudder. For the single rudder, flow visualization, time-mean velocity fields, and vorticity information were measured at angles of attack of 10°, 15°, 20°, and 25°. For the twin rudder, streamlines, instantaneous and time-mean velocity fields, vorticity, and turbulence intensity were measured and compared with each other to determine the critical distance between the single rudders. It was found that this critical distance should be less than 1.0C in order to decrease turbulence flow and vortices. [ABSTRACT FROM AUTHOR]

A one-dimensional depth-averaged velocity deformation model is developed to study the characteristics of the unsteady flows in open channels. The vertical profile of the streamwise velocity is expressed in a power series, and the coefficients of the velocity are evaluated by using the unsteady equation of motion. A concise form of the friction velocity for unsteady nonuniform flow is then proposed by utilizing the coefficients of the power series. The applicability of the model is validated with experimental data, producing reasonably good agreement between the model results and the observed data. Finally, the deformation of velocity at the surface is assured by comparing velocity distribution of numerical model with the computational results from a previous model. [ABSTRACT FROM AUTHOR]

When an unsteady free surface flow encounters an adverse slope, it results in a decelerating flow up the adverse slope. The time dependent turbulent flow is treated here by appropriately reducing the two-dimensional Reynolds averaged Navier-Stokes equation along with the equation of continuity considering turbulence closure. With suitable choice of parameters, the resulting differential equations are numerically solved to compute free surface and streamwise velocity profiles with time. It is found that initially the advancing free surface is convex upwards for a short time, followed by a jump of the free surface with a negative streamwise velocity that is a backwater rolling breaker due to deceleration of flow. At later time, however, the velocity becomes positive, that is, the breakers roll forward. This dual feature of motion, that is a surge followed by rolling breakers, is repeated for sometime before the jumps stop. The theoretical analysis presented here is motivated by tidal bores propagating upstream in an estuarine river. [ABSTRACT FROM AUTHOR]

This study is devoted to quantify the near-bed turbulence parameters in mobile-bed flows with bed-load transport. A reduction in near-bed velocity fluctuations due to the decrease of flow velocity relative to particle velocity of the transporting particles results in an excessive near-bed damping in Reynolds shear stress (RSS) distributions. The bed particles are associated with the momentum provided from the flow to maintain their motion overcoming the bed resistance. It leads to a reduction in RSS magnitude over the entire flow depth. In the logarithmic law, the von Kármán coefficient decreases in presence of bed-load transport. The turbulent kinetic energy budget reveals that for the bed-load transport, the pressure energy diffusion rate near the bed changes sharply to a negative magnitude, implying a gain in turbulence production. According to the quadrant analysis, sweep events in mobile-bed flows are the principal mechanism of bed-load transport. The universal probability density functions for turbulence parameters given by Bose and Dey have been successfully applied in mobile-bed flows. [ABSTRACT FROM AUTHOR]

In order to acquire the flow characteristics of polymer solution in the depressurization injection allocator, this paper to simulate analysis adopts numerical calculation method. Based on the Navier-Stokes equation, RNGk -- ε turbulent model and the power law fluid model, it establishes the annulus flow mathematical model of polymer solution in depressurization injection allocator and analyses velocity, pressure and turbulent kinetic energy distribution law of power law fluid in depressurization injection allocator. Pressure drop of annular allocator is greater than the spindle allocator's, viscosity loss less than the spindle allocator's. Finally, this paper obtains the law of flow field characteristics influencing on pressure drop and viscosity loss, and provides a theoretical basis for the optimization design of depressurization injection allocator in oil production. [ABSTRACT FROM AUTHOR]

This study focuses on the double-averaged (DA) turbulence characteristics and the local flow characterization in the flow zones over and within the flow-gravel-bed interface. The cumulative running averaged (CRA) value of a turbulence quantity for a given interval (in this study, half of the gravel size) of flow measurement is the prerequisite to determine a complete DA value of that quantity. The CRA turbulence quantities within the interfacial sublayer require a longer distance to become spatially independent. A method is proposed to fit the log law for the DA velocity over the bed and a polynomial law within the interfacial sublayer. The form-induced shear stress is 40% of the DA Reynolds shear stress occurring at the virtual bed level. The spatial velocity fluctuations diminish more rapidly than the temporal velocity fluctuations below the virtual bed level. In the inertial subrange of turbulent kinetic energy (TKE), the eddy size decreases significantly near the virtual bed level, and in the dissipation range, it increases slightly. The TKE fluxes directed downward and downstream compose the sweep events that are the prevailing mechanism within the interfacial sublayer. It is also confirmed by the quadrant analysis done for the bursting events. The form-induced energy fluxes below the crest level are comparable with the TKE fluxes. In the TKE budget, the form-induced diffusion rate is significant within the interfacial sublayer. The quantities constituting the TKE budget attain their peaks above the virtual bed level and reduce sharply below it. However, the pressure energy diffusion rate induces suction near the crest. The local flow characterization is done by separating the local velocity profiles into log, S-shaped, and accelerated classes. Unlike the velocity profiles per class, the profiles of turbulence quantities per class are almost indistinguishable within the interfacial sublayer. A comparison of the results of this study with those of previous studies shows more dissimilarities than similarities. [ABSTRACT FROM AUTHOR]

Abstract: Employing nanofluids is a novel way to enhance heat transfer. However, the micro-flow enhancement due to the adding of nanoparticles, which is one of the key mechanisms behind heat transfer enhancement in nanofluids, still lacks experimental observation and a microscale-level understanding. The aim of this work was to experimentally research flow characteristics of nanofluids and to explore the microscopic mechanism for micro-flow enhancement by molecular dynamics simulation. Utilizing a wavy-walled tube, the experiment measured flow characteristics and took flow visualization photos of nanofluids in different flow regimes. In addition, internal flow characteristics of nanofluids were simulated by MD method and statistically analyzed. The measuring results for frictional resistance showed that compared to that of de-ionized water the frictional resistance of nanofluids increases slightly in laminar and transition flow regimes, and shows no obvious increase in turbulent regime, which indicates no additional pump work is needed and is advantageous to engineering application. The visualization experiment results showed at the same Reynolds number the adding of nanoparticles causes more homogeneous longitudinal mixing and enhances mass transferring of nanofluids, which indicates the micro-flow in nanofluids is enhanced. And the MD simulation results showed the microscopic mechanism for micro-flow enhancement is mainly because the irregular micro-motions of nanoparticles enhance momentum exchange and increase the intensity of turbulence. The present work suggests the mechanism of micro-flow enhancement in nanofluids, which is the basis of proposing suitable theory to mathematically describe flow of nanofluids. [Copyright &y& Elsevier]

The effect of a vegetation patch (VP) on the flow and turbulence characteristics is studied. Two turbulence models are used to simulate patch presence. Numerical results for both mean and turbulence flow characteristics are compared with available measurements to assess the performance of these models for dense and sparse submerged VPs. The mechanisms which control the turbulent development inside the VP are also discussed, and the patch effect on the downstream flow and turbulence is assessed. The results indicate that the patch length and the vegetation density dominate the turbulent development. An increase in the vegetation density causes acceleration of the development of flow velocity and turbulence characteristics inside the patch and increases the channel length downstream of the patch where its effect is significant. Also areas with increased and decreased values of bed shear stresses are observed at the initial part and at the end of the patch, respectively. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, numerical and experimental results of the heat transfer and flow characteristics of the horizontal spiral-coil tube are investigated. The spiral-coil tube is fabricated by bending a 8.00mm diameter straight copper tube into a spiral-coil of five turns. The innermost and outermost diameters of the spiral-coil are 270.00 and 406.00mm, respectively. Hot and cold water are used as working fluids. The k–ε standard two-equation turbulence model is applied to simulate the turbulent flow and heat transfer characteristics. The main governing equations are solved by a finite volume method with an unstructured nonuniform grid system. Experiments are performed to obtain the heat transfer and flow characteristics for verifying the numerical results. Reasonable agreement is obtained from the comparison between the results from the experiment and those obtained from the model. In addition, the Nusselt number and pressure drop per unit length obtained from the spiral-coil tube are 1.49 and 1.50 times higher than those from the straight tube, respectively. [Copyright &y& Elsevier]